1. Field of the Invention
The present invention relates to fuel cell apparatuses and a method of manufacturing fuel cell apparatuses.
2. Description of the Related Art
Energy per volume that fuel cell apparatuses can generate is likely to become several to ten times larger than that of conventional cells, and the fuel cell apparatuses allow small-sized electric devices such as cell-phones and notebook PCs to be continuously used for a long period of time by filling the fuel cell apparatuses with fuel. Thus, the fuel cell apparatuses are expected to be used.
In a fuel cell, a fuel electrode having a catalyst and an oxidizer electrode having a catalyst are disposed on opposing faces of an electrolyte membrane, respectively. Fuel such as hydrogen gas is supplied to the fuel electrode, and an oxidant such as oxygen gas is supplied to the oxidizer electrode. These reactants react electrochemically through the electrolyte membrane therebetween.
When currents and voltages are measured while changing an external load on the fuel cell, a current-voltage curve as shown in FIG. 11 is obtained. In FIG. 11, Voc is called an open-circuit voltage and Isc is called a short-circuit current. As shown in the drawing, the output voltage decreases with an increase in the output current. Therefore, the output power has a peak in respect to the output current. Since the fuel cell generally has an electromotive force of only about 1 V, a plurality of fuel cells electrically connected in series with each other are used for driving an electric device.
The fuel cells are electrically connected in series with each other to make a vertical fuel cell stack by stacking the fuel cells so as to interpose conductive separators therebetween or to make a planar fuel cell stack by connecting the fuel cells arrayed on one plane surface with special wiring.
FIG. 12 is a drawing showing a structure of a conventional fuel cell apparatus. The fuel cell apparatus 121 supplies electric power to an electric device 124 by using the above-mentioned fuel cell stack 122. The fuel cell apparatus 121 includes various control means 123 in order to obtain stable output power. For example, the control means 123 include a pump and a valve for stably supplying fuel to the fuel cell stack, a fan for controlling operative temperature of the fuel cell stack, and a battery for aiding start of the fuel cell stack. These control means 123 are electrically connected in parallel with the fuel cell stack 122 and are operated by using a part of the power generated by the fuel cell stack.
Unevenness in power generation characteristics of the fuel cells of the fuel cell apparatus may occur on the basis of a method for arranging the fuel cells or a method for supplying an oxidant or fuel. For example, in Journal of Power Sources, 2002, 112: 410-418, F. B. Prinz, et al. have reported that unevenness in power generation characteristics occurs in a planar fuel cell stack. In other words, the above-mentioned short-circuit current of the fuel cells is different from that of each other. The causes are not clear, but it is pointed out that the short-circuit current decreases in the fuel cells arranged at the downstream of the oxidant or fuel and that there are some problems of pressure loss, reactant depletion, and moisture control.
If the fuel cell stack having such unevenness generates power at a level higher than a short-circuit current of any one of the fuel cells, the current may flow in the reverse direction in that fuel cell, which will deteriorate the power generation characteristics of the fuel cell. In this regard, it is believed that a voltage of power generated by other fuel cells is applied to the fuel cell and, as a result, electrolysis of water occurs in the fuel cell. In order to obtain stable output, the fuel cell apparatus is not operated in such a manner that the output current exceeds a short-circuit current of each fuel cell.
As described above, if the unevenness in the power generation characteristics of the fuel cells occurs in the fuel cell apparatus having the fuel cells electrically connected in series with each other, the output current of the fuel cell stack must be determined depending on the fuel cell of the smallest short-circuit current. Therefore, the power generation characteristics of each fuel cell cannot be sufficiently utilized. Furthermore, since a part of the output power from the fuel cell stack is supplied to the control means, power generation density of the fuel cell apparatus decreases, which is a problem.